1. Field of the Invention
The present invention relates generally to an uplink communication apparatus and method in a mobile communication system. More particularly, the present invention relates to an apparatus and method for transmitting and receiving a Random Access CHannel (RACH) in a mobile communication system.
2. Description of the Related Art
A 3rd Generation (3G) mobile communication system that operates in Wideband Code Division Multiple Access (WCDMA) based on the European mobile communication systems, Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), or Universal Mobile Telecommunication Service (UMTS) provides mobile users or computer users all around the world with a uniform service of transmitting packet-based text, digitized audio or video data, and multimedia data at or above 2 Mbps.
Particularly, the UMTS system uses a transport channel, i.e., an Enhanced uplink Dedicated CHannel (E-DCH), attempting to improve the performance of uplink packet transmission from User Equipments (UEs) to a Node B (or Base Station (BS)). To support more stable high-speed data transmission, the E-DCH adopts Adaptive Modulation and Coding (AMC), Hybrid Automatic Repeat reQuest (HARQ), Node B-controlled scheduling, and a shorter Transmission Time Interval (TTI).
AMC is a scheme for increasing the use efficiency of resources by determining a modulation scheme and a coding scheme for a data channel according to the channel status between a Node B and a UE. A combination of modulation scheme and a coding scheme is commonly referred to as a Modulation and Coding Scheme (MCS). Many MCS levels can be defined according to the supported modulation and coding schemes. As an MCS level is selected adaptively according to the channel status between a Node B and a UE, AMC increases the use efficiency of resources.
HARQ is a retransmission technique for compensating an erroneous packet when an initially transmitted data packet has errors. HARQ techniques can be categorized into Chase Combining (CC) and Incremental Redundancy (IR). In CC, a packet of the same format as an initial transmission packet having errors is retransmitted, whereas in IR, a packet having a different format from that of an initial transmission packet having errors is retransmitted. To increase the data rate, HARQ utilizes an N-channel Stop And Wait (SAW).
More specifically, according to N-channel SAW, a transmitter transmits different data during first to Nth TTIs and during (N+1)th to 2Nth TTIs, determines whether to retransmit the transmitted data or transmit new data depending on the reception of an ACKnowledgment/Negative ACKnowledgment (ACK/NACK) for the transmitted data. Each set of N TTIs are processed by independent HARQ processes and an (N+i)th TTI is referred to as an ith HARQ process, where N is an integer larger than 0 and the variable i indicating an HARQ process number is a natural number ranging from 1 to N.
Regarding data transmission on the E-DCH, a Node B determines if the uplink data transmission is available and, when it is available, the Node B determines the highest available data rate for the uplink transmission. The Node B transmits the determined information as a scheduling grant to a UE. Thereafter, the UE determines the data rate of the E-DCH based on the scheduling grant and transmits data at the data rate. This is commonly referred to as Node B-controlled scheduling.
A TTI is a basic transmission unit of packet data. Use of a 2-ms TTI shorter than the shortest 10-ms TTI used in legacy systems reduces a retransmission time delay and consequently, increases system throughput.
In the UMTS system, a time-domain transmission unit is represented in slots or frames. A 2-ms subframe is defined by three slots and a 10-ms frame is defined by five subframes. Therefore, the 2-ms E-DCH TTI corresponds to one subframe and the 10-ms E-DCH TTI corresponds to one frame.
FIG. 1 illustrates uplink packet transmission on E-DCHs in a conventional wireless communication system.
Referring to FIG. 1, reference numeral 100 denotes a Node B supporting E-DCHs and reference numerals 101 to 104 denote UEs using the E-DCHs. Herein, the terms BS and Node B are interchangeably used in the same sense. The UEs 101 to 104 transmit data on E-DCHs 111 to 114 to the Node B 100. The Node B 100 collects information about the buffer occupancy state, requested data rates, or channel statuses of the UEs 101 to 104 and performs a scheduling operation by determining the availability of E-DCH data transmission and an E-DCH data rate for each UE. Thereafter, the Node B 100 transmits scheduling grants to the individual UEs 101 to 104. The scheduling is done such that lower data rates are allocated to remote UEs, for example, the UEs 103 and 104, and higher data rates are allocated to nearby UEs, for example, the UEs 101 and 102, within a target noise rise or Riser over Thermal (RoT) of the Node B 100 in order to increase overall system performance. The UEs 101 to 104 determine their maximum allowed data rates for E-DCH data transmission according to the scheduling grants, determine E-DCH data rates within the maximum allowed data rates according to their buffer occupancy states, and transmit E-DCH data at the determined data rates.
Because uplink signals from different UEs are not synchronized and thus are not mutually orthogonal, they often interfere with one another. As the Node B receives more uplink signals, interference to an uplink signal from a particular UE increases, thereby decreasing the reception performance of the uplink signal. To overcome this problem, the uplink transmit power of the UE may be increased, which then interferes with other uplink signals, decreasing their reception performances. Consequently, the total power of uplink signals that the Node B can receive with acceptable reception performance is limited. RoT represents the uplink radio resources available to the Node B, and is defined as shown in Equation (1).RoT=Io/No  (1)
In Equation (1), Io denotes a power spectral density over a total reception band, i.e., the total power of all uplink signals received at the Node B, and No denotes the thermal noise power spectral density of the Node B. Therefore, a total allowed RoT, i.e., the total uplink radio resources available to the Node B, is limited to a predetermined value or below.
The total RoT is expressed as the sum of inter-cell interference, voice traffic, and E-DCH traffic. Because Node B-controlled scheduling prevents simultaneous transmission of packets at high data rates from UEs, the reception RoT of the Node B can be maintained at or below a target RoT, thereby ensuring acceptable reception performance all the time. That is, when high data rates are allocated to particular UEs, they are not allowed for other UEs in Node B-controlled scheduling. Consequently, the reception RoT does not exceed the target RoT, thus preventing the degradation of system performance.
FIG. 2 is a flowchart illustrating a conventional operation for transmitting and receiving an E-DCH.
Referring to FIG. 2, a Node B and a UE establish an E-DCH in step 202. The E-DCH setup involves exchanging messages on dedicated transport channels. In step 204, the UE transmits scheduling information to the Node B. The scheduling information includes UE transmit power information such as uplink channel information, information about extra available transmit power of the UE, and an amount of transmission data buffered in a UE buffer.
Upon receipt of scheduling information from a plurality of UEs communicating with the Node B, the Node B performs scheduling based on the scheduling information in step 206. More specifically, the Node B receives information transmitted for uplink transmission from the UEs and schedules the UEs based on the received information.
In step 208, the Node B transmits scheduling grants to UEs to which the Node B decides to grant uplink packet transmission. The scheduling grants may indicate increase/keep/decrease of maximum allowed data rates for the UEs through an E-DCH Relative Grant CHannel (E-RGCH), or may indicate maximum allowed data rates and allowed transmission timings through an E-DCH Absolute Grant CHannel (E-AGCH).
In step 210, the UE determines a Transport Format (TF) of an E-DCH according to the scheduling grant. The UE then simultaneously transmits TF information with transmitting uplink packet data on the E-DCH to the Node B in steps 212 and 214. The TF information includes an Enhanced Transport Format Combination Indicator (E-TFCI) indicating resource information needed to demodulate the E-DCH. The UE selects an MCS level, taking into account the maximum allowed data rate allocated by the Node B and its channel status and transmits the uplink packet data using the MCS level in step 214. A physical layer channel, i.e., an E-DCH Dedicated Physical Control CHannel (E-DPCCH), carries the E-TFCI information and a physical layer channel, i.e., an E-DCH Dedicated Physical Data CHannel (E-DPDCH), delivers the uplink packet data. Along with the E-DPDCH/E-DPCCH, a Dedicated Physical Control CHannel (DPCCH) is also transmitted, for use in channel estimation and power control of the Node B.
In step 216, the Node B determines if the TF information and the packet data have errors and generates an ACK/NACK signal according to the determination. If at least one of the TF information and the packet data has errors, the Node B transmits a NACK signal to the UE on an E-DCH HARQ Indicator CHannel (E-HICH), and if none of the TF information and the packet data have errors, the Node B transmits an ACK signal to the UE on the E-HICH in step 218. After an ACK signal, the packet data transmission is completed and the UE transmits new user data on the E-DCH. However, after a NACK signal, the UE retransmits the same packet data to the Node B on the E-DCH.
In the above-described operation illustrated in FIG. 2, if the Node B can receive scheduling information, such as the buffer occupancy states and power states of UEs from the UEs, it can allocate lower data rates to remote UEs, UEs in poor channel status, or UEs having lower-priority transmission data, or can allocate higher data rates to nearby UEs, UEs in good channel status, or UEs having higher-priority transmission data in order to increase overall system performance.
Typically, the RACH is used for signaling from a UE to a Node B. For example, the UE uses the RACH to register to a network after a power-on, to update its location information, or to initiate a call. Therefore, the RACH should have a relatively low data rate and wide cell coverage. Because the RACH is transmitted without a call connected to the UE, the UE has no knowledge of a necessary transmit power value. Accordingly, the UE roughly adjusts the transmit power value required for the RACH transmission by open-loop power control. The RACH includes a RACH preamble for initial access and a RACH message for data transmission. The Node B uses an Acquisition Indicator CHannel (AICH) as a response channel for the RACH preamble.
FIG. 3 illustrates a conventional physical layer RACH transmission procedure.
Referring to FIG. 3, the UE is first aware of RACH transmission resources including RACH access slots being RACH transmittable periods and signatures for UE identification on a Broadcast CHannel (BCH). The UE randomly selects a RACH access slot and a signature from among the RACH transmission resources and determines an initial RACH transmit power level by applying a predetermined offset to a measurement of a received downlink channel. The UE transmits a RACH preamble 312 including the selected signature at the determined initial RACH transmit power level in the selected RACH access slot. In FIG. 3, the initial RACH preamble 312 is initially transmitted at time t1 304. When receiving the RACH preamble 312 without errors, the Node B feeds back the signature included in the RACH preamble 312 as an ACK signal on an AICH. On the contrary, if failing to receive the RACH preamble 312 from the UE, the Node B does not transmit the AICH to the UE and the UE retransmits an RACH preamble 314 at a power level higher than the transmit power of the initial RACH preamble 312 by a predetermined value in an available RACH access slot.
In FIG. 3, the retransmitted RACH preamble 314 is infinitivally transmitted at time t2 306. The Node B notifies the UE that it has succeeded in receiving the RACH preamble 314 by transmitting an AICH 316 at time t3 308. Upon receipt of the AICH 316, the UE transmits intended data in a RACH message 318 at time t4 310. A time spacing tp-p 320 between the RACH preambles 312 and 314, a time spacing tp-a 322 between the RACH preamble 314 and the AICH 316 corresponding to the RACH preamble 314, and a time spacing tp-m 324 between the RACH message 318 and the previous RACH preamble 314 are pre-defined, i.e., known to both the Node B and the UE.
With the recent introduction of the E-DCH to the RACH, active research is underway to support a service requiring a periodic connection or a higher data rate than the conventional RACH, such as a HyperText Transfer Protocol (HTTP) request or Voice over Internet Protocol (VoIP) service. Accordingly, a need exists for specifying a RACH transmission procedure to support the service on the RACH.